Yama: Check for pid death before checking ancestry

BugLink: https://bugs.launchpad.net/bugs/1837477
commit 9474f4e7cd71a633fa1ef93b7daefd44bbdfd482 upstream.

It's possible that a pid has died before we take the rcu lock, in which
case we can't walk the ancestry list as it may be detached. Instead, check
for death first before doing the walk.

Reported-by: syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com
Fixes: 2d514487faf1 ("security: Yama LSM")
Cc: stable@vger.kernel.org
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 8298e094f4f7fa6338814902f3c6067b276a6032..7d5541c6a2251c5b09bade0674cda220d5fa3eaa 100644 (file)
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -373,7 +373,9 @@ static int yama_ptrace_access_check(struct task_struct *child,
                        break;
                case YAMA_SCOPE_RELATIONAL:
                        rcu_read_lock();
-                       if (!task_is_descendant(current, child) &&
+                       if (!pid_alive(child))
+                               rc = -EPERM;
+                       if (!rc && !task_is_descendant(current, child) &&
                            !ptracer_exception_found(current, child) &&
                            !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
                                rc = -EPERM;